In 2008, there were an estimated 12.7 million cases of cancer diagnosed worldwide and about 7.6 million deaths. The global cancer burden is growing at an alarming pace; in 2030 alone, about 21.3 million new cancer cases and 13.1 million cancer deaths are expected to occur, simply due to the growth and aging of the population. Cancer is the second most common cause of death in the US, exceeded only by heart disease, accounting for nearly 1 of every 4 deaths. The National Cancer Institute estimates that approximately 13.7 million Americans with a history of cancer were alive on Jan. 1, 2012. Some of these individuals were cancer free, while others still had evidence of cancer and may have been undergoing treatment. About 1,660,290 new cancer cases are expected to be diagnosed in the US in 2013. In 2013, about 580,350 Americans are expected to die of cancer, almost 1,600 people per day. Although medical advances have improved cancer survival rates, there is a continuing need for new and more effective treatment.
Kinase signaling pathways play a central role in numerous biological processes. Defects in various components of signal transduction pathways have been found to account for a vast number of diseases, including numerous forms of cancer, inflammatory disorders, metabolic disorders, vascular and neuronal diseases (Gaestel et al. Current Medicinal Chemistry (2007) 14:2214-2234). In recent years, kinases that are associated with oncogenic signaling pathways have emerged as important drug targets in cancers.
The cell division cycle, which regulates the transition from quiescence to cell proliferation, also involves various protein kinases that are frequently overexpressed in cancer cells. Because of their important role in the cell division cycle, such cell cycle kinases have also been explored as targets for cancer therapy.
One kinase associated with an oncogenic signaling pathway is the mammalian/mechanistic target of rapamycin (mTOR), which is a serine/threonine protein kinase that regulates cell growth, translational control, angiogenesis and/or cell survival. mTOR is encoded by the FK506 binding protein 12-rapamycin associated protein 1 (FRAP1) gene and is the catalytic subunit of two distinct protein complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2).
mTORC1 function is involved in many growth-related processes such as protein translation, ribosome biogenesis, transcription, autophagy and hypoxic adaptation. mTORC1 is best known as a key regulator of protein translation via its ability to phosphorylate the eukaryotic translation initiation factor 4EBP1, and S6 kinase (Hidalgo, M. J Clin Onc (2012) Vol 30, 1).
To date mTORC2 has best been described to regulate two major cell functions, including regulation of Akt and cell cycle-dependent organization of the actin cytoskeleton. mTORC2 phosphorylates Akt on serine 473 (Ser473) in its C-terminal hydrophobic motif, which, in conjunction with PDK1-mediated phosphorylation of threonine 308 (Thr308), confers full activation of Akt (Sarbassov D. D., et al. Science (2005) 307:1098-1101). mTORC2 regulates the actin cytoskeleton through an unclear mechanism which is rapamycin insensitive (Jacinto E., et al. Nat Cell Biol. (2004) 6: 1122-1128). Interestingly, mTORC2 phosphorylates PKC and SGK1 (serum- and glucocorticoid-induced protein kinase 1), and has also been implicated in controlling cell size (Ikenoue T., et al. EMBO J. (2008) 27: 1919-193; Rosner M., et al. Hum Mol Genet., (2009) 18: 3298-3310).
The mTORC1 and mTORC2 complexes are often distinguished by their ability to differentially bind and be inhibited by rapamycin and its analogs (rapalogs), which is in contrast to catalytic inhibitors of mTOR that can equally inhibit mTORC1 and mTORC2. Rapamycin inhibits mTOR by associating with its intracellular receptor FKBP12. The FKBP12-rapamycin complex then binds directly to the FKBP12-Rapamycin Binding (FRB) domain of mTOR enzyme (Jacinto E., et al. Cell (2006) 127: 125-137). As such rapamycin and rapalogs can be considered as allosteric inhibitors regulating the activity of mTORC1 only. Furthermore this regulation can be considered incomplete as the ability of these inhibitors to suppress 4EBP1 phosphorylation (an important downstream effect of mTORC1 inhibition) is considered to be only partial (Hidalgo, M. J Clin Onc (2012) Vol 30, 1).
Examples of cell cycle kinases include the Aurora kinases, first identified in yeast (Ipl1), Xenopus (Eg2) and Drosophila (Aurora). (Embo J (1998) 17, 5627-5637; Genetics (1993) 135, 677-691; Cell (1995) 81, 95-105; J Cell Sci (1998) 111(Pt 5), 557-572). In humans, three isoforms of Aurora kinase exist, including Aurora A, Aurora B and Aurora C (Carmena M, Earnshaw W C. Nat Rev Mol Cell Biol. (2003) 4:842-54). Aurora A and Aurora B play critical roles in the normal progression of cells through mitosis, whereas Aurora C activity is largely restricted to meiotic cells.
The Aurora A gene (AURKA) localizes to chromosome 20q13.2 which is commonly amplified or overexpressed at a high incidence in a diverse array of tumor types. (Embo J (1998) 17, 3052-3065; Int J Cancer (2006) 118, 357-363; J Cell Biol (2003) 161, 267-280; Mol Cancer Ther (2007) 6, 1851-1857; J Natl Cancer Inst (2002) 94, 1320-1329). Increased Aurora A gene expression has been correlated to the etiology of cancer and to a worsened prognosis. (Int J Oncol (2004) 25, 1631-1639; Cancer Res (2007) 67, 10436-10444; Clin Cancer Res (2004) 10, 2065-2071; Clin Cancer Res (2007) 13, 4098-4104; Int J Cancer (2001) 92, 370-373; Br J Cancer (2001) 84, 824-831; J Natl Cancer Inst (2002) 94, 1320-1329). This concept has been supported in experimental models, demonstrating that Aurora A overexpression leads to oncogenic transformation. (Cancer Res (2002) 62, 4115-4122; Mol Cancer Res (2009) 7, 678-688; Oncogene (2006) 25, 7148-7158; Cell Res (2006) 16, 356-366; Oncogene (2008) 27, 4305-4314; Nat Genet (1998) 20, 189-193). Both in vitro and in vivo studies have demonstrated that Aurora A induces tumorigenesis through genome instability. The potential oncogenic role of Aurora A has led to considerable interest in targeting this kinase for the treatment of cancer.
Previous studies have shown that cell signaling cross-talk between Aurora A and other cellular proteins are essential for fully-transformed phenotypes. The cross-talk between Aurora A and the mTOR signaling pathway represents an example of this. In mouse mammary tumor virus (MMTV)-Aurora A transgenic mice, constitutive phosphorylation of mTOR Ser2448 and Akt Ser473 was discovered in developed mammary tumors (Wang X., et al. Oncogene (2006) 25: 7148-7158). Elevated phosphorylation of mTOR Ser2448 and Akt Ser473 in Aurora-A transformed cells suggests that Aurora-A can potentially regulate two mTOR pathways, mTORC1 and mTORC2. More evidence suggests that either or both of mTORC1 and 2 is important for Aurora-A induced transformation since mTOR inhibition can abolish transformed phenotypes induced by Aurora A (Taga M., et al. Int J Biol Sci (2009) 19: 444-450). Of note, mammary tumor development can be observed only after long latency in MMTV-Aurora A mice.
Given the importance of the protein kinases involved in signal transduction pathways and the cell division cycle, it would be beneficial if more effective treatment regimens, which target these kinases could be developed. In particular, combined treatment regimens could be helpful for patients suffering from disease conditions including proliferative disorders, autoimmune diseases, inflammatory diseases, fibrotic diseases and kidney diseases, and might potentially even decrease the rate of relapse or overcome the resistance to a particular anticancer agent sometimes seen in these patients.
There is thus a need for new cancer treatment regimens, including combination therapies.